89 Formula 350 speedometer reading way off
Thread Starter
Member
Joined: Oct 2012
Posts: 104
Likes: 22
From: Glendale, AZ
Car: 1983 LU5 Trans Am, 1967 Chevelle
Engine: LU5, LSx
Transmission: 700R4, 4L60E
Axle/Gears: 3.73, 3.73
89 Formula 350 speedometer reading way off
The speedo in my 89 Formula 350 is reading much higher than what it should. I’m running the factory 9-bolt rear with 3.27 gears, stock 245/50R16 tires, and the correct speedometer gears (17 drive/45 driven). It reads around 55mph when I’m going 40. The odometer is obviously way off as well. The only thing that has been changed is the PROM, to a Hypertech. I’ve had 13 thirdgens and never had this happen, at least not while running the correct speedo gears. Does anyone have any ideas or have any suggestions as to what I can check?
Last edited by 1983LU5TA; Nov 28, 2017 at 02:23 PM.
Joined: Jul 2013
Posts: 1,208
Likes: 153
From: *member since 1999, I think - just can't remember my old name, and the big site crash...*
Car: 89 GTA ASC Conv., Prev: 89 GTA 6.3L
Engine: 5.7L L98 TPI
Transmission: 700r4 Automatic
Axle/Gears: 3.27:1 w/ JG1 Options:B2L, N10, U1A
Re: 89 Formula 350 speedometer reading way off
When did the issue begin? And what might have changed right before?
Thread Starter
Member
Joined: Oct 2012
Posts: 104
Likes: 22
From: Glendale, AZ
Car: 1983 LU5 Trans Am, 1967 Chevelle
Engine: LU5, LSx
Transmission: 700R4, 4L60E
Axle/Gears: 3.73, 3.73
Re: 89 Formula 350 speedometer reading way off
The speedo has been reading incorrectly since I got the car. It makes zero sense to me. I’ve swapped gears in TPI cars before and the speedo always read correctly after swapping the speedo gears. Everything is right. Even the Hypertech PROM (159311) is correct for the car
Last edited by 1983LU5TA; Nov 28, 2017 at 02:23 PM.
Joined: Oct 2001
Posts: 12,232
Likes: 1,166
From: Il
Car: 1989-92 FORMULA350 305 92 Hawkclone
Engine: 4++,350 & 305 CIs
Transmission: 700R4 4800 vig 18th700R4 t56 ZF6 T5
Axle/Gears: 3.70 9"ford alum chunk,dana44,9bolt
Re: 89 Formula 350 speedometer reading way off
I would swap in a different cluster and retest.
Thread Starter
Member
Joined: Oct 2012
Posts: 104
Likes: 22
From: Glendale, AZ
Car: 1983 LU5 Trans Am, 1967 Chevelle
Engine: LU5, LSx
Transmission: 700R4, 4L60E
Axle/Gears: 3.73, 3.73
Re: 89 Formula 350 speedometer reading way off
Can i swap in a 120 speedo for testing purposes? I dont know of any 140 speedometers to swap in place of mine that are local